| Chains, like sprockets, clutch plates, tyres, brake pads, oil and fuel are considered to be 'consumable' items on bikes, so I don't think it's reasonable to expect dealers to take responsibility for them.
If you've got a brand fire new bike and something like this fails then you've got a point, but more often than not we're not talking about new bikes here, we're talking about bikes that may be on their second or third chain.
It's also impossible for a dealer to be responsible for the way that an individual takes care of their bike, and impossible to say what life you should expect from any of these items. After all, somebody who cleans, lubes and adjusts their chain religiously and has a scottoiler keeping it lubed whatever the weather might have a chain in better nick at 15,000 miles than somebody who leaves their bike out in the rain, rides through all weathers and has never lubed the chain or checked it's adjustment would have in 5000 miles. So if that persons chain was shagged in 5K miles would it be the dealers problem?
There are obviously grey areas, but you can bet that anybody whose general lack of attention to machine maintainance had contributed to premature failure would do their utmost to cover their tracks.
There are always exceptions to this and there are well known makes of chain that have rightly or wrongly got themselves a reputation as snappers. I know that I wouldn't buy one given that reputation - I'll get a Tsubaki thanks and oil it before every ride (unless I'm just nipping to the shops)  | 
Jools
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